Dose rate effects in the antioxidant action in polypropilene

Due to its economic importance, radiation induced oxidation of polypropylene (hereafter referred to as PP) has been studied by various methods to predict the stability of medical equipment sterilized with ionizing radiation. It is well documented that the deterioration proceeds via oxidation initiated by radiation. However, the mechanism is still not clear, because of complicated effects such as inhomogeneous oxidation, dependence of polymer morphology, and the effects of additives. Stabilization of PP to high energy radiation can be achieved by controlling the radical reactions as well as the oxidation reactions. The oxidation reactions can be controlled by dissolving one or more stabilizers into polymer products; synergistic effects are observed as well. These stabilizers not only protect PP during irradiation, but also provide a necessary and important long-term stability during the subsequent storage period. The large body of knowledge concerning polymer antioxidants is preeminently empirical and lacks the understanding of reaction kinetics and mechanisms. Stabilization of PP to high energy radiation induced oxidation is even more complex in the case of nuclear track membranes. The two diverse processes taking place in irradiated polymer during and after irradiation should be controlled and balanced; first, to promote a scission and increase the radical concentration in heavy ion tracks in order to enable and speed up the hole making process and second, to prevent oxidation in the unirradiated portion of polymer film in order to preserve good mechanical properties of the membrane. In order to solve this problem, detailed knowledge of the radiation induced reactions in irradiated PP and PP's chemistry of aging has to be gained. (author)